Followers.

Because thin solid walls of brick or block may offer poor resistance to the penetration of wind driven rain, many loadbearing walls are built with two leaves of brick or block separated by a cavity, whose prime purpose is as a barrier to rain penetration.

Because the resistance to the passage of heat of a cavity wall by itself is poor, it is necessary to introduce a material with high resistance to heat transfer to the wall construction.

Most of the materials, thermal insulators, that afford high resistance to heat transfer are fibrous or cellular, lightweight, have comparatively poor mechanical strength and are not suitable by themselves for use as part of the wall structure. The logical position for such material in a cavity wall, therefore, is inside the cavity.